What does the R-factor measure?

• A. An insulation's resistance to heat flow, or conduction
• B. The electrical resistance of the insulation
• C. The compressive strength of the insulation
• D. The color stability of the insulation

Answer: (A)

R-factor measures how well insulation resists heat flow, primarily by conduction. A higher R-value means better resistance, so less heat transfers through the material. The value depends on the insulation material and its thickness, and in the United States is commonly expressed in ft²·°F·hr/BTU (or m²·K/W in SI units). The other properties listed—electrical resistance, compressive strength, and color stability—do not describe heat transfer, so they don’t reflect the effectiveness of insulation in reducing heat flow.